If I have a parallel beam of light parallel to x-axis, with the speed of light varying from bottom to top. Let it decrease from bottom to top. (To get such a beam one can pass the beam through a glass slab normal to its surface, whose refractive index increases linearly from bottom to top). Now although the beam is still parallel to x-axis, the wavefront is tilted. Will a convex lens (principle axis along x) be able to converge the rays at focus? There exists a path difference between two successes ice rays. Hence shouldn't we consider interference? Will there be a bright spot on a screen kept at focal plane but above the focus?